Such a walking assistance system has already been proposed in Japanese Patent Application No. 2001-109046 related to an application of the present applicant. This walking assistance system includes an electric actuator fitted to user's leg joints (i.e., a hip joint and a knee joint ), and the actuator is operated by supplying electric power from a power source within a backpack carried by the user on the back so as to generate a joint torque for extending/bending each joint, thus assisting the user in carrying out a movement such as walking.
Furthermore, a reduction gear in which a plurality of planetary gear mechanisms are axially stacked in multiple stages so as to achieve a large reduction ratio with a small size is known from Japanese Patent Application Laid-open No. 8-247225 and Japanese Patent Application Laid-open No. 11-37226.
The above-mentioned conventional walking assistance system includes an actuator integrated with a motor and a reduction gear, and since the reduction gear is required to have a large reduction ratio, there is the problem that the overall dimensions of the actuator increase. As a result, it becomes difficult to fit the actuator under a user's clothing, and since the actuator is exposed outside the clothing, the appearance is poor.
Although the above-mentioned conventional reduction gear, in which the plurality of planetary gear mechanisms are axially stacked in multiple stages, is excellent in having a large reduction ratio, since the axial dimensions are large, it is not suitable for use in an actuator of a walking assistance system.